An ultralow-density porous ice with the largest internal cavity identified in the water phase diagram

Abstract

Significance Among 18 known ice structures, ice XVI and XVII were produced by emptying the guest atoms/molecules encapsulated in cavities of porous ices. We demonstrate simulation evidence that the ultralow-density porous EMT ice (named according to zeolite nomenclature) is thermodynamically stable under negative pressures. Such a low-density solid (∼60% of the mass density of ice XVI) can be exploited for hydrogen storage with H2 mass density of 12.9 wt %, which is more than twice that (5.3 wt %) achieved by sII clathrate hydrate. With EMT ice, the temperature–pressure phase diagram of water under negative pressures is reconstructed. Like ice XVII, EMT ice could be produced by pumping off guest molecules in EMT hydrates preformed at high pressure. The recent back-to-back findings of low-density porous ice XVI and XVII have rekindled the century-old field of the solid-state physics and chemistry of water. Experimentally, both ice XVI and XVII crystals can be produced by extracting guest atoms or molecules enclosed in the cavities of preformed ice clathrate hydrates. Herein, we examine more than 200 hypothetical low-density porous ices whose structures were generated according to a database of zeolite structures. Hitherto unreported porous EMT ice, named according to zeolite nomenclature, is identified to have an extremely low density of 0.5 g/cm3 and the largest internal cavity (7.88 Å in average radius). The EMT ice can be viewed as dumbbell-shaped motifs in a hexagonal close-packed structure. Our first-principles computations and molecular dynamics simulations confirm that the EMT ice is stable under negative pressures and exhibits higher thermal stability than other ultralow-density ices. If all cavities are fully occupied by hydrogen molecules, the EMT ice hydrate can easily outperform the record hydrogen storage capacity of 5.3 wt % achieved with sII hydrogen hydrate. Most importantly, in the reconstructed temperature–pressure (T-P) phase diagram of water, the EMT ice is located at deeply negative pressure regions below ice XVI and at higher temperature regions next to FAU. Last, the phonon spectra of empty-sII, FAU, EMT, and other zeolite-like ice structures are computed by using the dispersion corrected vdW-DF2 functional. Compared with those of ice XI (0.93 g/cm3), both the bending and stretching vibrational modes of the EMT ice are blue-shifted due to their weaker hydrogen bonds.